STUDIES ON PROTEOLYTIC ACTIVITY AND FUNCTION OF THE THYROID GLAND IN RATS ADMINISTERED EXCESS IODIDE

: Thyroid auto-proteolytic activity in rats increased significantly only after several weeks of treatment with 1 mg iodide per day. Iodide added in vitro did not activate thyroid auto-proteolysis, nor was inhibition observed. Thyroid proteolytic activity was also tested using 131I-labelled heat-inactivated thyroid homogenate from control rats as substrate. Significantly increased proteolytic activity was again demonstrated in thyroid glands from rats on excess iodide for several weeks. The increased proteolytic activity is therefore probably not due to a decreased proteolytic resistance of thyroglobulin in the iodide-treated rats. Increased synthesis or decreased degradation of thyroid proteolytic enzymes is a more likely explanation for this phenomenon. These findings are at variance with those of some previous studies showing decreased proteolytic activity in thyroid glands from rats administered excess iodide. Since the rats remained euthyroid the increased auto-proteolysis, which occurred quite late, was considered an unexpected finding and it is at present not certain whether this is related to the successful adaptation. Treatment with iodide did not alter the thyroid and hypophyseal weight or histology. Serum free thyroxine level and thyrotropin concentration were also unaffected by excess iodide. The adaptation of the imtact rat thyroid to prolonged and excess iodide intake seems to occur without the assistance of an altered thyrotrophin secretion.